In recent years, in order to cope with the global warming, it is sincerely desired that the amount of carbon dioxide be reduced. In the automobile industry, expectations are centered on reduction of an emission amount of carbon dioxide by introduction of the electric vehicle (EV) and the hybrid electric vehicle (HEV). And development of an electric device such as a secondary battery for driving a motor, the electric device serving as a key for practical use of these vehicles, is assiduously pursued.
In particular, a lithium ion secondary battery is considered to be suitable for an electric vehicle since an energy density thereof is high and durability thereof against repeated charge/discharge is high. A capacity increase of the lithium ion secondary battery tends to further advance, and it becomes more and more important to ensure safety thereof.
In general, the lithium ion secondary battery has a configuration, in which a positive electrode having a positive electrode active material and the like coated on both surfaces of a positive electrode current collector and a negative electrode having a negative electrode active material and the like coated on both surfaces of a negative electrode current collector are connected to each other through an electrolyte layer having an electrolytic solution or an electrolyte gel held in a separator, and the positive electrode and the negative electrode are housed in a battery case.
As the separator, for example, a polyolefin microporous membrane with a thickness of approximately 20 to 30 μm is frequently used. However, in a case where the polyolefin microporous membrane as described above is used, there is a possibility that there may occur heat shrinkage owing to a temperature rise in the battery and occur a short circuit following this heat shrinkage.
Therefore, in order to suppress the heat shrinkage of the separator, a separator with a heat-resistant insulation layer, in which a heat-resistant porous layer is laminated on a surface of a microporous membrane of resin, is developed. For example, International Publication No. 2007/066768 describes that the heat shrinkage owing to the temperature rise in the battery is suppressed by using such a separator for a wound-type lithium ion battery.